This invention relates to the recovery of copper from copper bearing sulphide minerals.
Commercial bioleach plants which are currently in operation treating sulphide minerals, typically operate within the temperature range of 40xc2x0 C. to 50xc2x0 C. and rely on sparging air to the bioleach reactors to provide the required oxygen. Operation at this relatively low temperature and the use of air to supply oxygen, limit the rate of sulphide mineral oxidation that can be achieved. For example carrolite and enargite are relatively slow leaching at temperatures below 50xc2x0 C., and treatment at or below this temperature would result in poor and sub-economic metal extraction.
The use of high temperatures between 50xc2x0 C. and 100xc2x0 C. greatly increases the rate of sulphide mineral leaching.
The solubility of oxygen is however limited at high temperatures and the rate of sulphide mineral leaching becomes limited. In the case of using air for the supply of oxygen, the effect of limited oxygen solubility is such that the rate of sulphide mineral leaching becomes dependent on and is limited by the rate of oxygen transfer from the gas to the liquid phase.
The bioleaching of secondary copper bearing sulphide minerals is similarly problematic and to the applicant""s knowledge no commercial copper bioleach plants are in operation.
More particularly chalcopyrite has long been known to be generally refractory to bioleaching using mesophiles. A major challenge is the leaching of chalcopyrite, on an industrial scale, using thermophilic microorganisms.
The invention provides a method of recovering copper from a copper bearing sulphide mineral slurry which includes the steps of:
(a) subjecting the slurry to a bioleaching process,
(b) supplying a feed gas which contains in excess of 21% oxygen by volume, to the slurry, and
(c) recovering copper from a bioleach residue of the bioleaching process.
The method may include the step of pre-leaching the slurry prior to the bioleaching process of step (a). The pre-leaching may be effected using an acidic solution of copper and ferric sulphate.
The method may include the step of removing ferric arsenate from the bioleach residue before step (c). The ferric arsenate may be removed by precipitation.
The bioleach residue may be subjected to a neutralisation step which produces carbon dioxide which is fed to the feed gas of step (b), or directly to the slurry.
In step (c) copper may be recovered using a solvent extraction and electrowinning process. Oxygen which is generated during the copper electrowinning may be fed to the feed gas of step (b), or directly to the slurry.
Raffinate, produced by the solvent extraction, may be supplied to at least one of the following: the bioleaching process of step (a), and an external heap leach process.
Oxygen generated during the electrowinning process may be fed to the feed gas of step (b), or directly to the slurry.
The said slurry may contain at least one of the following: arsenical copper sulphides, and copper bearing sulphide minerals which are refractory to mesophile leaching.
The slurry may contain chalcopyrite concentrates.
As used herein the expression xe2x80x9coxygen enriched gasxe2x80x9d is intended to include a gas, eg. air, which contains in excess of 21% oxygen by volume. This is an oxygen content greater than the oxygen content of air. The expression xe2x80x9cpure oxygenxe2x80x9d is intended to include a gas which contains in excess of 85% oxygen by volume.
Preferably the feed gas which is supplied to the slurry contains in excess of 85% oxygen by volume ie. is substantially pure oxygen.
The method may include the step of maintaining the dissolved oxygen concentration in the slurry within a desired range which may be determined by the operating conditions and the type of microorganisms used for leaching. The applicant has established that a lower limit for the dissolved oxygen concentration to sustain microorganism growth and mineral oxidation, is in the range of from 0.2xc3x9710xe2x88x923 kg/m3 to 4.0xc3x9710xe2x88x923 kg/m3. On the other hand if the dissolved oxygen concentration is too high then microorganism growth is inhibited. The upper threshold concentration also depends on the genus and strain of microorganism used in the leaching process and typically is in the range of from 4xc3x9710xe2x88x923 kg/m3 to 10xc3x9710xe2x88x923 kg/m3.
Thus, preferably, the dissolved oxygen concentration in the slurry is maintained in the range of from 0.2xc3x9710xe2x88x923 kg/m3 to 10xc3x9710xe2x88x923 kg/m3.
The method may include the steps of determining the dissolved oxygen concentration in the slurry and, in response thereto, of controlling at least one of the following: the oxygen content of the feed gas, the rate of supply of the feed gas to the slurry, and the rate of feed of slurry to a reactor.
The dissolved oxygen concentration in the slurry may be determined in any appropriate way, e.g. by one or more of the following: by direct measurement of the dissolved oxygen concentration in the slurry, by measurement of the oxygen content in gas above the slurry, and indirectly by measurement of the oxygen content in off-gas from the slurry, taking into account the rate of oxygen supply, whether in gas enriched or pure form, to the slurry, and other relevant factors.
The method may include the step of controlling the carbon content of the slurry. This may be achieved by one or more of the following: the addition of carbon dioxide gas to the slurry, and the addition of other carbonaceous material to the slurry.
The method may extend to the step of controlling the carbon dioxide content of the feed gas to the slurry in the range of from 0.5% to 5% by volume. A suitable figure is of the order of 1% to 1.5% by volume. The level of the carbon dioxide is chosen to maintain high rates of microorganism growth and sulphide mineral oxidation.
The bioleaching process is preferably carried out at an elevated temperature. As stated hereinbefore the bioleaching rate increases with an increase in operating temperature. Clearly the microorganisms which are used for bioleaching are determined by the operating temperature and vice versa. As the addition of oxygen enriched gas or substantially pure oxygen to the slurry has a cost factor it is desirable to operate at a temperature which increases the leaching rate by an amount which more than compensates for the increase in operating cost. Thus, preferably, the bioleaching is carried out at a temperature in excess of 40xc2x0 C.
The bioleaching may be carried out at a temperature of up to 100xc2x0 C. or more and preferably is carried out at a temperature which lies in a range of from 60xc2x0 C. to 85xc2x0 C.
In one form of the invention the method includes the step of bioleaching the slurry at a temperature of up to 45xc2x0 C. using mesophile microorganisms. These microorganisms may, for example, be selected from the following genus groups: Acidithiobacillus (formerly Thiobacillus); Leptosprillum; Ferromicrobium; and Acidiphilium. 
In order to operate at this temperature the said microorganisms may, for example, be selected from the following species: Acidithiobacillus caldus (Thiobacillus caldus); Acidithiobacillus thiooxidans (Thiobacillus thiooxidans); Acidithiobacillus ferrooxidans (Thiobacillus ferrooxidans); Acidithiobacillus acidophilus (Thiobacillus acidophilus); Thiobacillus prosperus; Leptospirillum ferrooxidans; Ferromicrobium acidophilus; and Acidiphilium cryptum. 
If the bioleaching step is carried out at a temperature of from 45xc2x0 C. to 60xc2x0 C. then moderate thermophile microorganisms may be used. These may, for example, be selected from the following genus groups: Acidithiobacillus (formerly Thiobacillus); Acidimicrobium; Sulfobacillus; Ferroplasma (Ferriplasma); and Alicyclobacillus. 
Suitable moderate thermophile microorganisms may, for example, be selected from the following species: Acidithiobacillus caldus (formerly Thiobacillus caldus); Acidimicrobium ferrooxidans; Sulfobacillus acidophilus; Sulfobacillus disulfidooxidans; Sulfobacillus thermosulfidooxidans; Ferroplasma acidarmanus; Thermoplasma acidophilum; and Alicyclobacillus acidocaldrius. 
It is preferred to operate the leaching process at a temperature in the range of from 60xc2x0 C. to 85xc2x0 C. using thermophilic microorganisms. These may, for example, be selected from the following genus groups: Acidothermus; Sulfolobus; Metallosphaera; Acidianus; Ferroplasma (Ferriplasma); Thermoplasma; and Picrophilus. 
Suitable thermophilic microorganisms may, for example, be selected from the following species: Sulfolobus metallicus; Sulfolobus acidocaldarius; Sulfolobus thermosulfidooxidans; Acidianus infernus; Metallosphaera sedula; Ferroplasma acidarmanus, Thermoplasma acidophilum; Thermoplasma volcanium; and Picrophilus oshimae. 
The slurry may be leached in a reactor tank or vessel which is open to atmosphere or substantially closed. In the latter case vents for off-gas may be provided from the reactor.
According to a different aspect of the invention there is provided a method of recovering copper from a slurry containing copper bearing sulphide minerals which includes the steps of bioleaching the slurry using suitable microorganisms at a temperature in excess of 40xc2x0 C., controlling the dissolved oxygen concentration in the slurry within a predetermined range, and recovering copper from a bioleach residue.
The bioleaching may be carried out at a temperature in excess of 60xc2x0 C.
The dissolved oxygen concentration may be controlled by controlling the addition of gas which contains in excess of 21% oxygen by volume to the slurry.
Preferably the gas contains in excess of 85% by volume.
The bioleach residue may be subjected to a separation step to produce residue solids and solution and the copper may be recovered from the solution in any appropriate way, for example by means of a solvent extraction and electrowinning process.
The invention also extends to a method of enhancing the oxygen mass transfer coefficient from a gas phase to a liquid phase in a copper bearing sulphide mineral slurry which includes the step of supplying a feed gas containing in excess of 21% oxygen by volume to the slurry.
The feed gas preferably contains in excess of 85% oxygen by volume.
The invention further extends to a method of bioleaching an aqueous slurry containing copper bearing sulphide minerals which includes the steps of bioleaching the slurry at a temperature above 40xc2x0 C. and maintaining the dissolved oxygen concentration in the slurry in the range of from 0.2xc3x9710xe2x88x923 kg/m3 to 10xc3x9710xe2x88x923 kg/m3.
The dissolved oxygen concentration may be maintained by supplying gas containing in excess of 21% oxygen by volume to the slurry. The temperature is preferably in the range of from 60xc2x0 C. to 85xc2x0 C.
The invention further extends to a plant for recovering copper from a copper bearing sulphide mineral slurry which includes a reactor vessel, a source which feeds a copper bearing sulphide mineral slurry to the vessel, an oxygen source, a device which measures the dissolved oxygen concentration in the slurry in the vessel, a control mechanism whereby, in response to the said measure of dissolved oxygen concentration, the supply of oxygen from the oxygen source to the slurry is controlled to achieve a dissolved oxygen concentration in the slurry within a predetermined range, and a recovery system which recovers copper from a bioleach residue from the reactor vessel.
The oxygen may be supplied in the form of oxygen enriched gas or substantially pure oxygen.
The reactor vessel may be operated at a temperature in excess of 60xc2x0 C. and preferably in the range of 60xc2x0 C. to 85xc2x0 C.
The plant may include a pre-leaching stage for leaching the copper bearing sulphide mineral slurry before the slurry is fed to the reactor vessel. In the pre-leaching stage use may be made of an acidic solution of copper and ferric sulphate.
Various techniques may be used for controlling the supply of oxygen to the slurry and hence for controlling the dissolved oxygen concentration in the slurry at a desired value. Use may for example be made of valves which are operated manually. For more accurate control use may be made of an automatic control system. These techniques are known in the art and are not further described herein.
As has been indicated oxygen and carbon dioxide may be added to the slurry in accordance with predetermined criteria. Although the addition of these materials may be based on expected demand and measurement of other performance parameters, such as iron(II) concentration, it is preferred to make use of suitable measurement probes to sample the actual values of the critical parameters.
For example use may be made of a dissolved oxygen probe to measure the dissolved oxygen concentration in the slurry directly. To achieve this the probe is immersed in the slurry. The dissolved oxygen concentration may be measured indirectly by using a probe in the reactor off-gas or by transmitting a sample of the off-gas, at regular intervals, to an oxygen gas analyser. Again it is pointed out that measuring techniques of this type are known in the art and accordingly any appropriate technique can be used.
A preferred approach to the control aspect is to utilise one or more probes to measure the dissolved oxygen concentration in the slurry, whether directly or indirectly. The probes produce one or more control signals which are used to control the operation of a suitable valve or valves, eg. solenoid valves, automatically so that the supply of oxygen to an air stream which is being fed to the slurry is varied automatically in accordance with real time measurements of the dissolved oxygen concentration in the slurry.
Although it is preferred to control the addition of oxygen to a gas stream which is fed to the slurry a reverse approach may be adopted in that the oxygen supply rate to the reactor vessel may be maintained substantially constant and the rate of supply of the sulphide mineral slurry to the reactor vessel may be varied to achieve a desired dissolved oxygen concentration.
The invention is not limited to the actual control technique employed and is intended to extend to variations of the aforegoing approaches and to any equivalent process.
The method of invention is of particular benefit to chalcopyrite concentrates, which are more-or-less refractory to leaching at mesophile operating temperatures. The method of the invention therefore opens the door to commercial thermophile leaching of chalcopyrite which to the applicant""s knowledge was previously not possible. The added benefits of a high specific reactor sulphide oxidation duty and reduced specific power requirement for oxidation, while still advantageous, are of less significance in this instance.
Additionally copper bearing sulphide flotation concentrates frequently contain chalcocite and the method of the invention is of particular benefit, because chalcocite has a high leaching rate, even at typical mesophile operating temperatures, which is further increased at the higher temperatures used with moderate and extreme thermophiles. Thus the benefits of the invention, including a high specific reactor sulphide oxidation duty and reduced specific power requirement for oxidation, will be particularly beneficial during the bioleaching of copper bearing sulphide concentrates containing chalcocite, even at typical mesophile operating temperatures.
Copper may be recovered from solution by any appropriate process, for example solvent extraction followed by electrowinning, iron precipitation, or by resin-in-pulp applied to the slurry, followed by electrowinning.
If electrowinning is selected as the production method for copper, the oxygen generated at the anode in the electrowinning process may be used to supplement that used in the bioleach process, reducing the capital and operating costs required for oxygen production.
E PC